Touch screen panel (TSP) technology is embodied in various methods such as a resistive layer method, a capacitive method, and an infrared method. Thereamong, in the infrared method, a light emitting diode (LED) array and a pair of photodetector (PD) arrays are arranged outside a touch panel. Light emitted from the LED is hidden by a finger and so on when there is a touch input and, thus, coordinates when the light emitted from the LED is not detected by a PD may be calculated and a touch input may be detected.
The infrared method advantageously uses diversified touch elements such as a glove, a pen, and a stylus as well as a finger unlike in the capacitive method and so on. In addition, patterning is not necessarily performed on a touch panel glass and, thus, durability may be advantageously enhanced.
Recently, a structure configured to dispose a light emitter for emitting infrared rays and a light receiver for receiving emitted light at a lower surface of a panel to minimize a bezel portion has been widely used. In this case, light emitted from a lower surface of the panel may be emitted to an upper end of the panel using a light guide.
However, when light is emitted through the light guide, if a reflective surface of the light guide is smeared with water due to a finger touch and so on, frustrated total internal reflection (FTIR) but not total internal reflection (TIR) occurs.